Antilock braking systems (ABS) and traction control systems have become increasingly popular in both passenger cars and heavy duty vehicles. These systems require accurate wheel speed signals which vary in accordance with vehicle wheel speed. Such wheel speed signals are commonly generated by wheel speed sensors. These wheel speed sensors generally include a serrated tone wheel, which is mounted for rotation with the vehicle wheel, a sensing head, which is mounted to a non-rotating part of the vehicle adjacent to the tone wheel, and an interface circuit, which receives and conditions signals generated by the sensing head. The sensing head can consist of a coil wound around a cylindrical permanent magnet, the end face of which is in close proximity with the tone wheel. The tone wheel is usually made of a ferromagnetic material and is provided with serrations circumferentially spaced equally around the tone wheel. As the tone wheel turns in front of the sensing head, the flux density inside of the coil will change depending on whether the end face of the permanent magnet is facing a tooth or a serration defined by two adjacent teeth. This flux change induces an alternating voltage in the coil, the frequency of which is equal to the rate of change of the teeth and serrations of the tone wheel rotating past the end face of the magnet. Accordingly, the frequency of the output of the sensor is proportional to the vehicle wheel speed.
Since wheel speed sensors are critical components of ABS and traction control systems, there is a need for an wheel speed sensor interface circuit which tests the integrity of the speed sensors thus detecting faulty wheel speed signals. There is also a need for an interface circuit which includes built in safeguards for preventing faulty wheel speed signals. There is a further need for a wheel speed sensor which can be selectively tuned to accurately and efficiently detect wheel speeds in specific speed ranges for a variety of applications. There is a still further need for a multichannel, multiplexed interface circuit capable of conditioning and testing wheel speed signals generated by the multiple sensing heads included on a single vehicle.